Unless treated, aqueous systems are prone to undergo biological contamination. One of the most successful methods of preventing such contamination is by treating the aqueous system with an aqueous solution of chlorine dioxide.
Chlorine dioxide is typically generated on a continuous basis at the application site utilizing motive/dilution water and precursor co-reactant chemicals. The efficient generation of chlorine dioxide is obviously of economic and environmental significance. Generation efficiency is defined as the percentage conversion of precursor chemicals to chlorine dioxide. Poor generation efficiency results in lost profits and unconsumed precursor chemicals can lead to the generation of other by-products that potentially adversely affect the environment.
Generation efficiency is adversely affected for many reasons. It can be caused by the degradation of the sodium hypochlorite strength, which is a normal occurrence over time, or can result from the exposure of the sodium hypochlorite to heat and/or sunlight. Another cause for generation inefficiency is that the sodium hypochlorite may react with chemicals in the motive/dilution water, which results in the deactivation of the sodium hypochlorite. Variations in the motive/dilution water alkalinity also adversely affect the precursor chemical requirement, namely the amount of acid, which is required for pH control to achieve efficient chlorine dioxide generation.
Poor generation efficiency not only is uneconomical, but the by-products that result, when chlorine dioxide generation is not efficient, could potentially adversely affect human health and the environment. Whether the treated water is used as drinking water, or used for once-through or re-circulating cooling systems, there can be associated environmental issues. For example, the acceptable amount of chlorine dioxide in drinking water is typically limited to 0.8 mg/l; the amount of chlorite, (a precursor chemical) is typically limited to, for example, 0.8 mg/l; and the sum total of chlorite, chlorate, and chlorine dioxide (Total Residual Oxidant) is typically limited to, for example, 1 mg/l.
Additionally, if excess chlorine is used with chlorite in the generation of chlorine dioxide, then chloramines can be formed as a by-product. Excess chlorine may also result in the production of other unwanted by-products, such as trihalomethanes, halo acetic acids and halogenated organics in general.
Unwanted by-products, e.g. oxidants such as bromine, chlorine, monochloramines, monobromamines, hypochlorite, hypochlorous acid, chlorite, hypobromite, hypbromous acid, chlorine dioxide, chlorite, chlorate, and 1-bromo, 3-chloro-5,5-dimethlyhdrantoin can be hazardous when discharged into the environment in excessive quantities. Table I provides typical lethal doses of various oxidants for dalphina and rainbow trout.
TABLE IAquatic Lethality of Various Oxidants(Typical Oxidant Lethal Concentration, LC 50, mg/l)Daphnia 48 hourRainbow Trout 96 hourBromine0.311.07Chlorine0.020.13Hypochlorite as NaOCl0.061.0Hypochlorous Acid as Cl20.0270.045Hypobromous Acid as Br20.710.23Chlorine Dioxide as ClO20.29290Chlorite as ClO2—0.1641Chlorate as ClO3—316242001-Bromo-3-chloro-5,5-0.480.87dimethylhydantoin
In view of the data reported in Table I, if chlorine dioxide generation is inefficient, it will be necessary to detoxify the chlorine dioxide prior to its discharge into an environmental receiving stream, and then detoxify these unwanted oxidants from the chlorine dioxide treated waters.
Disposing of treated water, e.g. process water and drinking water, also is a problem if the generation of chlorine dioxide is inefficient. Inefficient generation results in higher concentrations of sodium and chloride ions in the treated water, which is often used to irrigate agricultural land. If the treated water contains high concentrations of sodium ions, chloride ions, and other ions, this adversely affects the ability of the soil to absorb and retain water. This may limit the number of gallons of treated waters that can be used to irrigate agricultural land each year, which reduces the amount of treated water that can be disposed of through irrigation.
It is known that the production of chlorine dioxide can be maximized and the formation of unwanted by-products can be minimized by generating chlorine dioxide electrolytically under vacuum, using a complex series of electrolytic cells separated by a semi permeable gas transfer membrane. See, for instance, WO 94/26670. When this method is used, the chlorine gas generated in the electrolytic cell to passes into the treatment stream, but the amount of hypochlorous acid and hypochlorite ion resulting from hydrolysis in the treatment stream is reduced. Unfortunately, overtime the efficiency of chlorine dioxide generation drifts away from the initial set-up value due to variability in motive water flow, motive water chemistry and/or variability in precursor active strength or concentration.
All citations referred to under this description of the “Related Art” and in the “Detailed Description of the Invention” are expressly incorporated by reference.